Field of the Invention
The field of the invention is ladder polymers.
Description of Related Art
Renewable energy and materials is a rapidly growing field, the development of which is in higher demand than ever. One major branch of renewable energy is organic electronics and semiconducting materials. Organic semiconductors made of conjugated small molecules have several advantages over their silicon-based counterparts including renewability, their ability to be solution-processed into lightweight and flexible films, and their ability to have their properties easily tuned through chemical synthesis.
Low band-gap materials based on fused, aromatic organic compounds are important to the development of organic photovoltaics (OPVs) as they can absorb the longer wavelengths of the solar spectrum that smaller aromatic units cannot. Band gaps can be reduced by increasing the planarity of the conjugated backbone by minimizing various steric interactions between aromatic units. Steric interactions tend to cause backbone twisting that result in a pi-electron conjugation that is only effective over shorter, segmented distances. This distance is affected by the structure of different compounds and inherently varies between different materials. As the conjugation length is increased, the band gap will decrease continually. However, conjugated organic materials will reach a point at which additional conjugated units will no longer result in a significant band gap decrease. In this way, increasing conjugation length quickly approaches an asymptotic value for the band gap.
One approach to synthesize highly planar, low band gap materials is by using polymeric systems that are fused through the majority of, if not the entirety of, a conjugated polymer (CP). A CP that is comprised entirely of fused rings is called a ladder polymer. Due to the reasons mentioned above, ladder polymers have very low band gaps. Further, an additional benefit of an extended fused ring system is better charge transport due to pi-stacking between adjacent polymer chains. One challenge associated with ladder polymers is poor solubility in organic solvents to remain solution-processable.